Europe Education Program - 'Catch A Star'
Project - "Magellanic Clouds"


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Astronomical observatory in Varna, Bulgaria



1. Mitko Slavov - teacher

2.Victor Vassilev - member

3.Boian Kukushev - member




1. Introduction and History

2. Science Information

3. Exercise: 'Estimating The Distance to Remote Stars and Galaxies Using The Relation 'Period - Luminosity'

4. Conclusion

5. Gallery

6. Reference

1. Introduction and History

The Magellanic Clouds are the two nearest galaxies to the Milky Way. They were called after the name of Magellan by the scrivener of his expedition - Pigaffetta. In January 1521 Pigaffetta recorded in his diary: 'On the South Pole can't be seen the same constellations as on the North Pole. Here there are two groups of small misty stars, which resemble little clouds, which are not much remote from each other'. Doubtless the Magellanic Clouds were known by the people, who lived in the south hemisphere. That is confirmed by some legends of the Australian aborigines, according to which the Clouds were a family couple or two fires.

The Magellanic Clouds can't be observed from the north hemisphere. The Large Cloud is positioned in the Dorado and Mensa constellations whilst the Small Cloud is in the Tucana constellation. Observed with the naked eye the Large Magellanic Cloud has visible diameter of about 6 degrees and the Small Cloud - 3 degrees. The visible distance between the clouds is 21 degrees.

2. Science

A) Position of the Magellanic Clouds on the Hubble's classification diagram

The Magellanic Clouds don't have regular shapes and they are classified as irregular galaxies (in visible rays). In infrared rays the clouds look like unstructured systems, resembling elliptical galaxies. In the Large Magellanic Cloud can be found a similarity of intersecting line (as in the SB-galaxies) and spirals.

This is the so-called "pitchfork" scheme of the galaxies according to Hubble. There you can see where the Magellanic Clouds are situated - irregular galaxies (Irr).


B) Position of the Magellanic Clouds compared to the Milky Way galaxy

The Magellanic Clouds are the two closest satellites to our galaxy. The Large Magellanic Cloud is at a distance of 160 000 ly and the Small Magellanic Cloud - 190 000 ly.

C) Position of the Magellanic Clouds in the Local Group

The Andromeda Galaxy, M31, is the biggest and most massive galaxy in the Local Group. Our Milky Way Galaxy takes second place. Scattered around these two primary galaxies are at least 25 smaller ones, most of which are dwarf elliptical. The illustration above shows an artist's rendition of the Local Group; the table lists these galaxies in order of increasing distance from Earth (the letter d preceding a Hubble type indicates a dwarf galaxy). The ten galaxies closest to us are satellites of the Milky Way Galaxy. Similarly, the Andromeda Galaxy has eight satellites. Astronomers occasionally find additional nearby dwarf galaxies that are also members of the Local Group.

We may never know the total number of galaxies in the Local Group because dust of the plane of the Milky Way obscures our view over a considerable region of the sky. Nevertheless, we can be certain that no additional large spiral galaxies are hidden by the Milky Way because radio astronomers would have detected 21- cm radiation from them, even though their visible light is completely absorbed by interstellar dust.

D) Comparison of The Magellanic Clouds and The Milky Way Galaxy

a) Resemblance:

The Magellanic Clouds comprise all known objects from our galaxy - stars, stellar clusters, and nebulae. Over 10000 variable stars, in the Magellanic Clouds, have been found, including a lot of Cepheids (variable stars from the delta Cep type). Similarly to the Milky Way galaxy, the globular stellar clusters in the Large Magellanic Cloud outline a significant spherical system and associations of hot blue giants - the Large Magellanic Cloud's galactic plane.


b) Differences:

  • Masses: If MS is solar mass, then MLMC = 10^10 MS, MSMC = 2,5.10^9 MS, while MGalaxy 10^12 MS.
  • Composition: The Magellanic Clouds mainly contain young hot stars and in the Galaxy the mass of these stars is only 10% of the Galaxy's mass.
  • Globular clusters: The globular stellar clusters in the Magellanic clouds consist of blue stars while in the Galaxy - of red stars.
  • Variable stars: The mean period of the Cepheids in the Milky Way galaxy is 6 days, in the Large Magellanic Cloud it is 4,3 days and in the Small Magellanic Cloud - 2,6 days. The longest period of Cepheids in the Galaxy is around 100 days, while in the Magellanic Clouds it reaches 200 days.
  • Evolution: The bigger relative part of hot stars in the Magellanic Clouds shows that the evolution of the stars, which formed the two Clouds, is slower in comparison to the stars of the Milky Way galaxy. The scientific explanation of these differences is that the clouds of gas and dust, which formed the two Clouds and the Galaxy billions of years ago, had different chemical composition.

E) Comparison Of The Large Magellanic Cloud and the Small Magellanic Cloud

The Large Magellanic Cloud contains mostly young and hot stars (Population I), but in its core there are old stars - yellow and red giants (Population II). In the Small Magellanic Cloud blue hot stars are concentrated in the central part of the Cloud. The Large Cloud has approximately 1200 globular stellar clusters and more than 100 planetary nebulae. In the Small Cloud there are 120 globular clusters and 350 nebulae. Because the mass of the Small Magellanic Cloud is smaller than the mass of the Large Cloud, evolution in the Small Cloud should be slower than in the Large Cloud. But it is known that the relative part of older objects in the Small Magellanic Cloud is more than this in the Large Cloud. That is result of the fact that the protocloud, which formed the Small Magellanic Cloud, had more heavy elements, compared to the protocloud that formed the Large Magellanic Cloud.

21-cm radio observations show us that both Magellanic Clouds are submersed in one and the same cloud formed of HII. From the Small Magellanic Cloud to the Milky Way galaxy there is a strip of HII - this is the so called Magellanic Stream (a similar strip exists between the Large Magellanic Cloud and the Galaxy too, but it is very frail). A possible reason for forming the Magellanic Stream is the gravitational influence of the Milky Way.

The Magellanic Clouds form a double galaxy i.e. they circle around common center of the masses. The radial velocities of the stars in the Large Magellanic Cloud are positive - +275 km/s, and the Small Cloud - +170 km/s, which means that they drift away from our galaxy. This determines their future as objects, moving away from us.


F) Significant objects in the Magellanic Clouds

a) Association 30 Dorado in the Large Magellanic Cloud

This association contains hundreds of blue giants. It is wrapped by a huge luminous nebula - NGC 2070 (the Tarantula Nebula). The mass of this nebula is approximately 5.10^6 MS and its size - 700 ly. On a picture in X-rays the Tarantula Nebula is presented with its diffuse twinkle in green and in the left low corner is the most powerful X-ray source in the Large Magellanic Cloud noted as X-1. X-1 is a double system, where the first component is an old star with very high density and the second is a giant with huge atmosphere. Another powerful X-ray source is the R136 object. In the beginning R136 was considered a cluster of about 1000 hot stars, after that it was determined R136 contained three objects and the most luminous of them was thought to be a superstar with mass approximately 2500 MS, and at last it was understood that R136 was a star cluster with incredible density and unbelievable small size.


b) The supernova in the Large Magellanic Cloud

On February 23, 1987 a supernova was discovered in the Large Magellanic Cloud. It was designated SN 1987A because it was the first to be discovered that year. With the study made on the supernova the model for supernovae was confirmed in general, because an increasing of the neutrino stream had been observed according to the theory for supernovae.

It turned out that SN 1987A is a unique star because:

  • Since 1604 that has been the single supernova, visible with the naked eye;
  • For the first time a supernova from II type was observed in an irregular galaxy;
  • According to the star evolution theory, only old red supergiants blow up as supernovae, but it was proved SN 1987A was the result of exploding of blue giant with T = 15000 K, diameter equal to 50 dS and luminosity equal to 10^4 LS (dS and LS - solar diameter and solar luminosity);
  • SN 1987A has a light echo, i.e. around it two concentrical rings are observed. They due to diffusing and reflection of the supernova's light by two nebulae, positioned between the Milky Way galaxy and SN 1987A.

G) Information Sources

For the reason that the Magellanic Clouds can not be seen from the north hemisphere we don't have our own observations so we used different information sources. This information was received by visual observations, observations with a telescope (H. Leavitt has used pictures of the Magellanic Clouds, made by a small telescope in Peru), and today observations made with the Hubble Space Telescope are used, photographic methods (discovered in the beginning of the 20th century), spectral analysis (using this analysis we can determine a galaxy's composition), radio astronomy (rapidly developed after World War II), photo- and electrophotometric methods (used to determine visible magnitudes of stars and galaxies), X-ray astronomy, and CCD receivers (with such receivers is noticed the light echo of SN 1987A ).

3. Exercise: 'Estimating the distance to remote stars and galaxies using the relation 'period - luminosity'

The only one direct method for measuring the distance to stars is the method of the year's parallax. But it can be used to determine

the distance only to the nearest stars (less than 1000 pc). For measuring bigger distances other methods are used. These methods are indirect and to be calibrated, the parallaxes' method is used.

Such an indirect but very important way of measuring is the using of the period-luminosity relation (relation P-L) in Cepheids. The Cepheids are variable stars from delta Cep type, which typical curve is shown on the graphic on the left. The history of this relation began in the beginning of the 20th century in the Harvard Observatory, when a collaborator of the observatory Henrietta Leavitt (1868-1921) found 1777 variable stars in the Magellanic Clouds. 969 from these stars were in the Small Magellanic Cloud. H. Leavitt determined that some of the stars with increasing their period the stars' luminosity increased too. She chose 25 variable stars from the Small Magellanic Cloud and in 1912 she published a diagram of this relation. Because distances between stars in a stellar system are much smaller than distance between our galaxy and the system, then we can accept the stars in the Small Cloud are at the same distance and so the brighter stars observed by H. Leavitt would have bigger luminosity compared to the darker. Consequently the examined relationship between the visible magnitudes (m) and the periods (P) is a result of relation between luminosity and periods. In 1913 Herzsprung, who was making a study on variable stars in the Milky Way, deducted that the variable stars, which Leavitt observed, were Cepheids like these in the Milky Way and so the P-L relation could be applied for all Cepheids. In such way distances to other galaxies can be measured as in our Galaxy because the Cepheids are giant stars and can be observed from large distances.

In the beginning of the 20th there was a great problem with calibration of the P-L relation because it was impossible a direct measuring the distance of at least one Cepheid. Today such a measuring has already been accomplished.

When using the P-L relation we can determine the distance by taking the basic equation:

(1)M = m + 5 - 5lgr

where M is the absolute magnitude of the star, m - visible magnitude, and r - the distance to the star.

From (1) we have

(2)lgr = (m - M + 5) / 5

==> r = 10^(m - M + 5) / 5

We can also use the relation between absolute magnitude M and luminosity L of the star, expressed in solar luminosity units (LS = 1):

(3)M = 4,8 - 2,5lgL

There are three ways of using this relation:

A) When we use the P - M (period - absolute magnitude) diagram

By given period P we can determine immediately M by observations we have the visible magnitude m and substitute them in (2) .


Example: Find the distance to a Cepheid which has a period P = 10 days and a visible magnitude m = 11.


Solution: Use the diagram to determine M = -4 and after a substitution in (2) we get:

r = 10 ^(11 + 4 + 5) / 5 pc = 10^4 pc = 10 kpc.


B) We can use also the P - L (period - luminosity) diagram

From the scheme by a given period the luminosity of the star is determined and when substituted in (3) , we get the absolute magnitude M.


Example: Determine the distance to a Cepheid with a period of changing the brightness P = 20 days and a visible magnitude m = 14,8.


Solution: From the diagram L = 10^4 LS we substitute in (3) and we get

M = 4,8 - 2,5lgL = -5,2

==> r = 10^(m - M + 5) / 5 pc = 10^5 pc = 100 kpc

C) The third way is by using the P - M analytic relation

M = -1,7 - 2,5lgP

Example: Calculate the distance to a Cepheid with a period P = 10 days and visible magnitude m = 5,8.

Solution: We substitute immediately

M = -1,7 - 2,5 = -4,2


r = 10^(m - M + 5) / 5 pc = 10^3 pc = 1 kpc

For people who like calculations, we give you the table below with the visible magnitude (m) and period (P) of five Cepheids from the Small Magellanic Cloud.


4. Conclusion

We have chosen the theme about the Magellanic Clouds because from Cepheid observations in the Small Magellanic Cloud was discovered the Period - Luminosity relation, which played a very significant part in solving such an important problem, which was determining the distance to remote galaxies and stars. Using the exercise we demonstrated how this can be done in a simple manner. Of course, the reality is far more complicated because Cepheids are not exactly homogenous class because the interstellar absorption was not noticed and the relation P - L should be relation P - L - T (T - temperature).

Our love to exotic objects pushed us to choose the Magellanic Clouds as our project. Doubtless observing the Magellanic Clouds is a beautiful dream for us but everyone must always believe that it will be fulfilled.


5. Gallery


The Magellanic Clouds
The photo above shows a small sector of the Large Cloud before the outburst, with the doomed star - B3 supergiant - identified by an arrow. The view below shows the supernova SN 1987A a few days after the explosion. (Anglo - Australian Observatory)
Above is the region N44 and below is DEM 192. In these images, red is Halpha emission, green is [SII], and blue is [OIII]. Notice the green area to the upper-left in the image of N44. This is a previously undetected (at least optically) supernova remnant!
Three nebular complexes in the Large Magellanic Cloud. To the left is N9 (the large loop), in the lower right is N5, and in the upper right is N4 complex. The image is combination of images made in three filters: Halpha emission, green is [SII], and blue is [OIII]. The greenish areas, such as the little hook just in the right edge of N9 and the large half bubble in N4, are newly identified supernova remnants.
The superbubble LMC 2 in the light of hydrogen (Halpha 6563, left) and sulfur ([SII] 6724, right).


6. Reference


(1) Universe / Wiliam J. Kaufmann III / New York / 1997

(2) Astrono,y of the 20th century / O. Struve and V. Zeburgs / 1968

(3) Galaxies show new secrets / N. Nikopov M. Kalinkov / Sofia / 1995

(4) Physics and astronomy / textbook for class 12 / V. Golev / Sofia / 2002


The images are from: